The degree of oxygenation of tumor cells and thus, their radiosensitivity is directly related to the blood flow. The degree of radiation-induced damage to normal tissue may also be related to vascular damage. The blood flow in the Walker 256 carcinoma, skin and muscle of SD rats will be measured at different time intervals after 2000 rads in a single exposure or in 4 or 8 fractions using the microsphere method. Implications of the vascular changes for the hypoxic fraction in this tumor would then be measured. Blood flow is believed to play an important role also for the differential effect of hyperthermia on tumors and normal tissues. This aspect has, however, not been fully understood. We will determine the effect of hyperthermia alone or in combination with radiation on the blood flow in the Walker tumor, skin and muscle of the rat. This information should enable us to understand the relationship between vascular change and reoxygenation kinetics in tumors during the course of radiotherapy. The study on the change in blood flow by hyperthermia alone or in combination with radiation will reveal the mechanism of therapeutic effect of hyperthermia and help us devise the best sequence of hyperthermia and radiotherapy.